Python instance attribute takes initial value from class attribute with the same name?

Question

class bambino(object):
    counter = 7
    def __init__(self):
        print("bambino.counter is self.counter ?", bambino.counter is self.counter)
        self.counter += 1
        print("bambino.counter is self.counter ?", bambino.counter is self.counter)

bambi1 = bambino()
print ("bambi1.counter:", bambi1.counter)
print ("bambino.counter:", bambino.counter)

prints:

bambino.counter is self.counter ? True
bambino.counter is self.counter ? False
bambi1.counter: 8
bambino.counter: 7

I understand that by doing self.counter += 1 counter becomes an attribute of the instance not of the class.

But why did bambi1.counter take it's initial value from bambino.counter?

Pavel Anossov · Accepted Answer · 2013-02-02 12:34:50Z

5

If an attribute is not found in an object, it gets looked up higher in the hierarchy, first in its class, and then, if not found, in the superclasses.

self.counter += 1 is equivalent to self.counter = self.counter + 1. So to assign the bambi1.counter, Python first needs to get the value of bambi1.counter. Since the bambi1 does not initially have a counter, python doesn't find it and has to look it up in its class.

PS: Please capitalize your class names.

answered Feb 2, 2013 at 12:34

Pavel Anossov

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4 Comments

ThiefMaster Over a year ago

Might be worth mentioning that the behavior is different for mutable objects when they are just modified and not reassigned.

Pavel Anossov Over a year ago

It's not the mutability per se, it's that some types have += overloaded to mutate the object in-place instead.

ThiefMaster Over a year ago

Ah I didn't even think of the += case but more generic cases like foo = {} in the class and then self.foo[..] = ..

Karl Knechtel Over a year ago

This isn't a problem if you already understand that mutating an object is fundamentally different from replacing it.

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